The present invention relates the field of creating structures within photosensitive waveguides such as those formed on the silicon wafers or the like.
The creation of complex structures within a waveguide utilizing UV exposure processing is becoming increasingly popular. Normally, the wafer is subjected to a series of UV processing steps so as to form the structures within the wafer. An example of such a process is illustrated in FIG. 1 which illustrates a section through a wafer which includes a substrate 2 on which is formed normally a first silicon dioxide layer 3 followed by a UV sensitive layer 4 which forms the core of the waveguide and which is in turn followed by a second silicon dioxide layer 5. Normally, UV rays eg. 6 are employed in a controlled manner so as to form structures within the core 4. These structures can include Bragg gratings etc.
Unfortunately, the idealized case of FIG. 1 does not eventuate in all circumstances. As illustrated in FIG. 2, often significant reflections occur at the boundary layer between the layers 2 and 3 such that significant reflections eg. 8 (up to 70% of the UV beam 6) are reflected so as to interfere with the formation of core structures within the photosensitive waveguide 4. As a result of this reflection process, significant loss of contrast is incurred which results in inferior results. Further, excessive UV light is often provided near the boundary layer and is likely to weaken the structure. Of course, damage at the substrate should be prevented. This damage is thought to affect the type of index change which is achieved at the core by changing the substrate stresses.
It is an object of the present invention to provide for the utilization of a UV absorbing layer so as to increase the contrast of structures written utilizing UV processing steps and to minimise damage at the substrate.
In accordance with a first aspect of the present invention, there is provided a waveguide having photosensitive properties comprising: a waveguide substrate; a first cladding layer formed on the waveguide substrate; a UV absorbing layer formed on the first cladding layer; a UV sensitive layer having optical transmission properties adapted to be changed with UV irradiation, the layer formed on or closely adjacent the UV absorbing layer; and a second cladding layer, being substantially UV transparent, on the UV sensitive layer.
Preferably, there is further provided a third cladding layer intermediate of the UV absorbing layer and the UV sensitive layer.
The UV absorbing layer can comprise a germanosilicate material. The absorbing layer can be adapted to change a physical property upon UV absorption. The UV absorbing layer can be of variable thickness, the thickness being in accordance with predetermined requirements.
In accordance with a further aspect of the present invention, there is provided a method of UV processing a photosensitive waveguide wherein the waveguide can comprise: a waveguide substrate; a first cladding layer formed on the waveguide substrate; a UV absorbing layer formed on the first cladding layer; a UV sensitive layer having optical transmission properties adapted to be changed with UV irradiation, the UV sensitive layer formed on or closely adjacent the UV absorbing layer; and a second cladding layer, being substantially UV transparent, on the UV sensitive layer; the method comprising the step of: UV irradiating only a predetermined portion of the UV sensitive layer.
The UV absorbing layer can be adapted to change its physical properties upon UV irradiation and the method further can comprise the step of: UV irradiating only a predetermined portion of the UV absorbing layer.